Near the end of the (2nd?) Shadow War, A Vorlon Armada was headed to obliterate the Centauri homeworld beacuse of the Centauri's association with the Shadows. At this time a large Shadow Armada was on the island of Celeini on Centauri Prime. They knew the Vorlons were coming and what would happen when they showed up, yet refused to leave- why stubbornly keep such a large fleet on a planet that's about to be destroyed?
The short answer is that the Shadows thought that the Vorlons would balk at destroying a large, populated planet just to get at the Shadow fleet. Mr Morden gives Londo an overview of their tactical thinking on the subject:
Morden: The Vorlons will never attack Centauri Prime. Small colonies, deep-range planets, sure. But to destroy a world as big as this? No. They don't have the will. - Falling Toward Apotheosis
As to whether he's actually correct, we'll never know. JMS spoke to this issue in an interview for the excellent Midwinter site:
JMS: Morden is living in denial...he's afraid, and hoping he's right. Centauri Prime has more civilians than any other planet or colony hit so far: three billion people. The shadows are hoping a figure that high will daunt even the Vorlons.
But he did point out that the gloves have definitely come off (on both sides):
Q. Wasn't the planet-killer a break in the rules?
JMS: That * followed * his trip to Z'ha'dum...so yes, the rules began to slip after that.